<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>数据类型概览</title>
</head>
<body>
<script>
    var a;
    console.log(typeof (a));  // undefined
    var b = 33;
    console.log(typeof (b));  // number
    var c = 'tincyho';
    console.log(typeof (c));  // string
    var d = [1, 2, 3];
    console.log(typeof (d));  // object
    var e = {name: 'tincyho', age: 30};
    console.log(typeof (e));  // object
    var f = true;
    console.log(typeof (f));  // boolean
    var g = null;
    console.log(typeof (g));  //object
    console.log(undefined == null);  // true
    console.log(undefined === null);  // false

    var str = 'abc', emptyStr = '';
    if(str){
        console.log("I'm here!");
    }
    if(emptyStr){
        console.log("u can't see me!");
    }
</script>
</body>
</html>